test_ukify: explicitly remove big temporary directories

test_ukify: explicitly remove big temporary directories

pytest intentionally keeps around a limited number of the previous test
temporary directories [1]. This is generally OK, but in our tests that generate
initrds, we create a few very large files (both the initrd and kernel in a few
copies), which quickly adds up. I had a particularly large initrd (because of
some mkosi-initrd shenanigans), and I unded up with dozens of gigabytes of
temporary files from the tests. Let's just nuke the dirs where we write
kernel data.

[1] https://github.com/pytest-dev/pytest/issues/543